Electrodeposition is one of the most widely used processes for applying metallic coatings on the surfaces of articles. Such metallic coatings are frequently applied in order to confer improved appearance, resistance to corrosion, resistance to wear, hardness, frictional properties, solderability, electrical characteristics, or other surface properties.
Electrodeposition processes entail deposition of a metal or alloy from a solution onto a surface of an article by electrochemical action driven by an electric current. Electrodeposition processes are carried out by contacting an electrically-conductive surface, termed the substrate surface, with a solution of one or more metal salts and passing an electric current through the solution to the surface. The substrate surface is thus made to form a cathode of an electrochemical cell. Metal cations from the solution are reduced at the substrate surface by electrons from the electric current so that a reduced metal or alloy deposits on the surface. The term "electrodeposition" refers both to electroplating processes, in which the deposited metal or alloy adheres to the substrate surface, and to electroforming processes, in which the deposited metal or alloy is detached from the substrate surface after it is deposited.
For an electrodeposition bath of a given composition, the microstructure, composition and other properties of the material deposited from the bath generally depend in part upon the characteristics of the electric current used in the electrodeposition process. An article by J. J. Avila and M. J. Brown in the November 1970 issue of Plating disclosed a pulse plating process for electroplating gold using an electric current which was rapidly pulsed on and off. In the pulse-plating process as described in the article, the current was switched on for a time sufficient to deposit the ions of gold in the electroplating bath adjacent to the cathode and was then switched off until the bath equilibrium was reestablished. According to the article, an advantage of electroplating gold with a pulsed current relative to plating with a conventional direct current stemmed from reducing concentration polarization at the cathode, which tended to eliminate hydrogen gas bubbles at the cathode. Hydrogen embrittlement was reduced and the gold deposit had a relatively high density and purity. The gold electroplated with the pulsed current was essentially homogeneous in structure and composition.
U.S. Pat. No. 3,886,053 (the '053 patent) disclosed a hone-forming process for electroplating chromium which involved simultaneously plating and machining the surface to be plated. The plating current was pulsed to control the hardness of the chromium. Specifically, the "on-time" period and "off-time" period of the pulsed plating current were initially selected to form soft, bonding plating at the junction of the chromium plating and the surface to be plated. Thereafter, the off-time period of the pulsed plating current was progressively reduced to increase the hardness of the plated chromium. The progressive reduction of the off-time continued until the off-time was reduced to zero near the end of the process, so that a maximum hardness was obtained at the wearing surface. According to the '053 patent, the gradual increase in hardness across the thickness of the plating avoided hydrogen embrittlement of the base metal and reduced tensile-stress adhesion failures of the plating.
An article by U. Cohen et al. in Journal of the Electrochemical Society, volume 130, pp. 1987-1995, disclosed a process for producing multilayered deposits of silver-palladium alloy for electrical contacts. The layers of the deposits were arranged in a cyclic sequence. Differences in thickness and composition between the individual layers were obtained by modulating the current to the cathode during electrodeposition. The authors reported that, other than a difference in brightness, a preliminary comparison between the cyclic multilayered silver-palladium alloy and silver-palladium alloys plated with a conventional direct current did not reveal any clear differences in tests relevant to the contact finish properties of the alloys.